To be used in road materials, asphalt must first meet certain specifications. For example, as a result of the Strategic Highway Research Program (SHRP), the Federal Highway Administration (FHWA) has developed a battery of tests and specifications for asphalt, known as the Superpave binder specification and test methods, designed to ensure that road materials have a longer lifetime before repair. Such specifications, referred to herein as the SHRP tests, may be categorized as comprising high, intermediate and low temperature Performance Grade (PG) tests. In addition, some agencies have adopted Compatibility tests to prevent separation of polymer and asphalt.
If a particular sample of asphalt produced in an oil refinery does not pass minimum specifications for all of the above-described SHRP tests, including compatibility testing, then the asphalt is not acceptable for use as road material. Various techniques have therefore been developed to alter the rheological properties of asphalt, so that it will meet the minimum requirements of the SHRP tests.
For example, a polymer, such as rubber, may be added to asphalt to produce polymer modified asphalt (PMA) in order to improve the asphalt's rheological properties so that the product meets specified limits at the required temperatures. PMA can also be used in asphalt emulsions, seal coats, joint fillers. PMA may also be used in other applications beyond paving materials, such as roofing asphalt and waterproofing materials or other material well known to those skilled in the art. Certain PMAs may fail the compatibility test, however. This, in turn, may necessitate further processing, such as cross-linking the polymer to thereby improve the PMA's compatibility test score. For example, a well-known means to introduce cross-links into PMAs containing rubber involves heating the rubber in the presence of a crossing linking agent, such as Sulfur. The cross linking process, commonly known as vulcanization, results in the production of sulfide bonds between the unsaturated double bond portions of rubber. The network of cross-linked rubber formed throughout the PMA improves the PMA's rheologic properties, and more importantly, improves the compatibility test score of the PMA.
In the setting of an asphalt production plant, the efficient and safe delivery of Sulfur to PMA may be problematic. The handling of large quantities of powdered Sulfur, for example, may present significant fire, explosion and health hazards to plant workers. Moreover, it may be difficult to mix powdered Sulfur into PMA in a uniform fashion. Alternatively, the Sulfur may be incorporated into an aqueous-based emulsion and then delivered to the PMA. Because such Sulfur containing aqueous emulsions are being delivered to hot asphalt and maintained at about 350° F. (177° C.), large amounts of steam are generated. The generation of steam laden with Sulfur-based compounds, such as hydrogen sulfide, generates health risks, thus necessitating an expensive and efficient venting system. Moreover, when aqueous emulsions contact asphalt, the asphalt will foam, thereby having a deleterious effect on the preparation of the PMA. Although the incorporation of Sulfur into an oil-based emulsion helps avoid the formation of steam and foam, other difficulties remain.
For example, the addition of powdered sulfur to the vapor space of a hydrocarbon tank results in free-floating dust which can stick to and build up on the interior surface of a hydrocarbon tank. Sulfur dust can also react with hydrocarbon vapors and cause further deposits on the interior surfaces of the tank. These deposits require periodic cleanout, can result in insoluble foreign matter in asphalt, and also represent a potential fire hazard. In addition, the slow mixing and dispersion of solid sulfur into PMA can result in localized high concentrations of sulfur resulting in excessive cross-link density and formation of insoluble material and/or gelling (rapid increase in viscosity) of the asphalt.
Furthermore, cross-linking agents, such as Sulfur separate out of certain aqueous or oil-based emulsions, and settles to form a sludge. Sludge formation, in turn, may impair the consistent delivery of cross-linking agent to PMA, resulting in the nonuniform formation of cross-links in the PMA. And, once sludge comprising the bulk of cross-linking agent forms at the bottom of aqueous or oil emulsions, it is extremely difficult to cost effectively resuspend the cross-linking agent. Thus, such emulsions may have undesirably short shelf lives (i.e., less than one day). This, in turn, burdens the costs and logistics of producing cross-linked PMA having acceptable performance grade and compatibility properties.
Accordingly, what is needed in the art is a polymer modified asphalt composition having uniform cross-linking facilitated by the efficient delivery of Sulfur to the asphalt, while not experiencing the above-mentioned problems.